Systems for manufacturing commercial products of rare earth magnet include a single part system wherein a part of substantially the same shape as the product is produced at the stage of press molding, and a multiple part system wherein once a large block is molded, it is divided into a plurality of parts by machining. These systems are schematically illustrated in FIG. 1. FIG. 1a illustrates the single part system including press molding, sintering or heat treating, and finishing steps. A molded part 101, a sintered or heat treated part 102, and a finished part (or product) 103 are substantially identical in shape and size. Insofar as normal sintering is performed, a sintered part of near net shape is obtained, and the load of the finishing step is relatively low. However, when it is desired to manufacture parts of small size or parts having a reduced thickness in magnetization direction, the sequence of press molding and sintering is difficult to form sintered parts of normal shape, leading to a lowering of manufacturing yield, and at worst, such parts cannot be formed.
In contrast, the multiple part system illustrated in FIG. 1b eliminates the above-mentioned problems and allows press molding and sintering or heat treating steps to be performed with high productivity and versatility. It now becomes the mainstream of rare earth magnet manufacture. In the multiple part system, a molded block 101 and a sintered or heat treated block 102 are substantially identical in shape and size, but the subsequent finishing step requires cutting. It is the key for manufacture of finished parts 103 how to cutoff machine the block in the most efficient and least wasteful manner.
Tools for cutting rare earth magnet blocks include two types, a diamond grinding wheel inner-diameter (ID) blade having diamond grits bonded to an inner periphery of a thin doughnut-shaped disk, and a diamond grinding wheel outer-diameter (OD) blade having diamond grits bonded to an outer periphery of a thin disk as a core. Nowadays the cutoff machining technology using OD blades becomes the mainstream, especially from the aspect of productivity. The machining technology using ID blades is low in productivity because of a single blade cutting mode. In the case of OD blade, multiple cutting is possible. FIG. 2 illustrates an exemplary multiple blade assembly 1 comprising a plurality of cutoff abrasive blades 11 coaxially mounted on a rotating shaft 12 alternately with spacers (not shown), each blade 11 comprising a core 11b in the form of a thin doughnut disk and an abrasive grain layer 11a on an outer peripheral rim of the core 11b. This multiple blade assembly 1 is capable of multiple cutoff machining, that is, to machine a block into a plurality of parts at a time.
For the manufacture of OD abrasive blades, diamond grains are generally bonded by three typical binding systems including resin bonding with resin binders, metal bonding with metal binders, and electroplating. These cutoff abrasive blades are often used in cutting off of rare earth magnet blocks.
When cutoff abrasive blades are used to machine a rare earth magnet block of certain size into a plurality of parts, the relationship of the cutting part (axial) width of the cutoff blade is crucially correlated to the material yield of the workpiece (magnet block). It is important to maximize a material yield and productivity by using a cutting part with a minimal thickness, machining at a high accuracy to minimize a machining allowance and cutting sludge, and increasing the number of parts available.
In order to form a cutting part with a minimal width (or thinner cutting part) from the standpoint of material yield, the cutoff wheel core must be thin. In the case of OD blade 11 shown in FIG. 2, its core 11b is usually made of steel materials from the standpoints of material cost and mechanical strength. Of these steel materials, alloy tool steels classified as SK, SKS, SKD, SKT, and SKH according to the JIS standards are often used in commercial practice. However, in an attempt to cutoff machine a hard material such as rare earth magnet by a thin OD blade, the prior art core of alloy tool steel is short in mechanical strength and becomes deformed or bowed during cutoff machining, losing dimensional accuracy.
One solution to this problem is a cutoff wheel for use with rare earth magnet alloys comprising a core of cemented carbide to which high hardness abrasive grains such as diamond and cBN are bonded with a binding system such as resin bonding, metal bonding or electroplating, as described in JP-A H10-175172. Use of cemented carbide as the core material mitigates buckling deformation by stresses during machining, ensuring that rare earth magnet is cutoff machined at a high accuracy. However, if a short supply of cutting fluid is provided to the cutting part during machining of rare earth magnet, the cutoff wheel may give rise to problems like glazing or loading even when a core of cemented carbide is used, which problems increase the machining force during the process and induce chipping and bowing, providing a detrimental impact on the machined state.
Approaches to address this problem include arrangement of plural nozzles near the cutoff blades for forcedly feeding cutting fluid to the cutting parts and provision of a high capacity pump to feed a large volume of cutting fluid. The former approach is quite difficult to implement in combination with a multiple blade assembly comprising a plurality of blades arranged at a close spacing of about 1 mm because nozzles cannot be arranged near the blades. In the latter approach of feeding a large volume of cutting fluid, the air streams created around the cutting parts during rotation of the cutoff blades cause the cutting fluid to be divided and scattered away before it reaches the cutting parts. If a high pressure is applied to the cutting fluid to forcedly feed it, the pressure is detrimental to high-accuracy machining because it causes the cutoff blades to be bowed and generates vibration.